1. FIELD OF THE INVENTION
The present invention relates to scan signal processing, and more particularly to a system and a method of converting an analog scan signal having a substantially common shading profile to a digital signal.
2. DESCRIPTION OF THE PRIOR ART
When an analog signal is converted to a digital signal such as bi-level or tri-level signal, the analog signal voltage is compared with a reference voltage or reference voltages to produce the digital signal. The conversion precision depends on the precisions of the analog voltage and the reference voltage.
In a facsimile or an optical character reader (OCR), optical information, such as characters on a document sheet, is scanned by photo-electric converter means, such as a CCD line sensor or a proximity line sensor, to produce an analog video signal, which is then converted to a digital video signal. The analog video signal may comprise a background component representing a plain paper component and the information component superposed on the background component. When the optical information representing the darkness and brightness on the document sheet is converted to a bi-level video signal, the conversion precision depends on the background component, e.g. the distribution of illumination on the document sheet, image focusing conditions of an optical system including lenses and distribution of sensitivity of individual sensor elements in a line sensor. Thus, even if the object (document sheet) is plain (e.g. uniformly white), the scanned analog video signal (referred to as a blank or white scan signal) corresponding to the background component may not have a constant voltage level but may have a specific profile. This profile is reproduced in each scan with substantially the same pattern. Such a characteristic distribution of the analog video scan signal representing the background component (state of no information) is called shading. A general or relative characteristic distribution is called a shading profile, and a characteristic distribution including voltage level is called a shading characteristic, in this specification. When the analog information signal containing shading is compared with a constant reference voltage to convert it to a digital signal, a correct digital information signal is not produced.
Heretofore, optical compensation and electric compensation for shading have been proposed. In the optical compensation, the illumination of the document sheet may be adjusted by regulating or adjusting a light source or disposing a filter or a mask so that a white scan signal having a constant signal level may be produced. This method needs a complex and large device and takes a long time for adjustment and yet is hard to attain a sufficient effect particularly for compensation of nonuniformity in sensitivity among picture elements in a line sensor.
The electrical compensation includes controlling the gain of a video amplifier to produce a shading-free video signal and effecting compensation in the signal processing for converting an analog signal containing shading to a digital signal.
It has been proposed, as a form of electric compensation, to store the shading in a memory and read it out in a scan cycle to modify the reference or slice signal to compensate the shaded signal.
Co-pending U.S. patent application Ser. No. 151,982 teaches to carry out shading compensation using a stored shading profile and a peak voltage of the scan signal held in a peak-hold circuit.
Co-pending U.S. patent application Ser. No. 260,090 further proposes normalization of the shading characteristic by the peak value thereof and denormalization of the stored shading profile by an instantaneous peak value, so as to improve the conversion accuracy.
These techniques are, however, not fitted for compensating rapid or steep changes in the shading profile with a simple circuit configuration of a small memory capacity.